For structuring hard-to-etch substances, such as gold, lift-off methods or methods for the structured application of a coating material on a material structure are employed in semiconductor technology, for example, when processing wafers. Here, photoresist, such as negative resist, is applied on a material structure, such as a multi-layer construction, a substrate, or a wafer, and then partially exposed. After exposing the photoresist, baking the material structure, on which the photoresist has been applied, may take place, in order to strengthen the cross-linkage of the negative resist. After baking, the negative resist is developed. Then, a coating material is applied on the material structure with the structured resist layer over the entire area, whereupon the negative resist is removed, and along with it the coating material layer applied thereon. In the areas in which the photoresist has not been removed, the material structure is not coated with the coating material. In the lift-off method, a positive resist may also be used. Here, the exposed areas are dissolved out or removed by the developer.
For developing the negative resist, an aqueous solution of TMAH or tetra methyl ammonium hydroxide material may be applied. This solution is highly alkaline, with the pH ranging from 13.5 to 14, so that corrosion may occur in the solution during the developing process when contacting metal on the material structure by the developer. This corrosion increasingly occurs when a stack of different metals is in direct contact with each other and at the same time has contact to the developer through the procedural sequence. One result of the corrosion is an attack on contact pads applied on the wafer, for example. As a result of the corrosion, these have diminished bondability and also often are not suited as under-bump metallization. The destruction of the contacts pads by the corrosion thus leads to reduced fabrication yield.
The corrosion of contact pads on the wafer is especially disadvantageous in the production of bulk acoustic wave filters or BAW filters, which are employed in mobile telephones. There, when measuring the frequency, which is still performed in industrial fabrication methods advantageously on wafer level, the stacks are contacted with a conducting measurement tip, in order to check the frequency response of the BAW filters, which often leads to the needle piercing metal layers in the contact pad and the metals in the contacting region blending with each other. The metals blended with each other form a plurality of corrosion elements, among which the less noble metal is dissolved during the ensuing development process, i.e., developing the negative resist.
Apart from aqueous developers, polar organic developers not having the above problem also exist. Independently of the developer used, it is desirable if a protruding edge develops at the periphery of the cross-linked part of the resist, because this provides for the tear-off edge in the coating material applied on the structured resist, and through which the solvent can dissolve the cross-linked part in the subsequent step, and hence lift off the coating material located thereon.